


How Things Could Be

by mildlyproductivetrashbag



Category: Ao no Exorcist | Blue Exorcist, Kuroshitsuji | Black Butler
Genre: Alternate Universe, Crossover, Familiars, Faustian Contract - Freeform, Gen, Rin is a dork, Sebastian is a demon king, Summoning
Language: English
Status: Completed
Published: 2019-04-28
Updated: 2019-04-28
Packaged: 2020-02-09 07:52:49
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,209
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/18633955
Author URL: https://archiveofourown.org/users/mildlyproductivetrashbag/pseuds/mildlyproductivetrashbag
Summary: When Rin attends the class for summoning familiars, he accidentally summons Sebastian. By the time he first encounters Amaimon in the forest, this is what happens.The demon king of earth learns that there are demons even he shouldn't dare to piss off.





	How Things Could Be

**Author's Note:**

> This is something I've had on the back of my mind for a really long while. Keep in mind, I haven't read or seen Ao no Exorcist in literally about a year and a half, so the details in this fic might be a bit spotty at best. I only reached the start of the Lucifer arc in the manga, back when I used to read it so please bear with me. I just wanted to get the idea out there, so. Yeah. Here it is.
> 
> If anyone is interested in this story (which I sincerely doubt) then I might continue it. But right now, I'm pretty happy with where I left it!

When he saw Shiemi in Amaimon's arms, something inside him broke. A hot, burning, anger, flared from within him, and he saw red.   
  
He was shaking, hands trembling from the sheer rage coursing through his veins. Already, he could feel his flames, his power, trying to push themselves past the Seal in response to his emotions, but he tamped them down. Forcefully. Now was not the time for him to expose himself and make matters worse. He had other means of taking care of the green bastard.  
  
"Sebastian!" He called out, and the mark on his collarbone flared and began to emit a red glow. The demon in question appeared beside him with a pleasant smile.  
  
"Master," Sebastian said, one corner of his mouth turning ever so slightly upwards, so that his expression more closely resembled a smirk than a smile. "It seems you've gotten yourself in a rather," he glanced at Amaimon and wrinkled his nose, "unpleasant situation, I see."   
  
"Yeah, yeah, get on with it!" Rin pointed towards Amaimon, who had gone frozen with shock at the sight of Sebastian, for a reason Rin had yet to fully understand. "Help Shiemi, and," His resolve wavered slightly, still unsure of their arrangement. But he steeled himself and forged on. "Get rid of him."  
  
"Is that an order, master?" Sebastian asked slyly. The question was a joke. A mocking statement. The two of them knew Rin didn't give orders lightly. Sebastian's eyes were already glowing in anticipation.  
  
Rin glared at him. With a sudden motion he pulled the collar of his shirt down and revealed the Faustian contract burnt into his flesh. A hint of fear slipped into Amaimon's eyes, and Rin felt a sense of vicious satisfaction.  
  
"Sebastian," Rin said, "I order you to get rid of that demon, and bring Shiemi back safely." This time, his voice was firm and confident, belying none of the anger and anxiety bubbling underneath the surface.   
  
"Yes, master." Sebastian answered, and with a feral grin, he lunged towards Amaimon. As he jumped, his features shifted to take on a form more befitting his demonic nature. His nails lengthened, morphing into claws sharp enough to be used as a deadly weapon. Moving faster than the other exwires could follow, he tried to attack Amaimon.   
  
The demon king was quick enough to react, jumping out of the way before Sebastian's claws could reach him. But his movements were hampered by the girl still in his arms. In the seconds before Sebastian attempted to attack him again, he made a decision. When he moved to jump away once more, he threw Shiemi to the side.  
  
The exwires gasped collectively, all instinctively moving forwards in an attempt to catch her. But only Rin actually succeeded, leaping forwards with his arms outstretched, and catching her in midair. They both landed heavily, but Rin took the brunt of the impact. His chin slammed down onto hard stone with an audible crack, making almost everyone in the vicinity wince. Shiemi, though still dazed, moved out of his grip and did her best to help him.  
  
Meanwhile, Sebastian and Amaimon were circling each other, albeit with very different approaches. Amaimon did his best to keep his face as blank as it always was, but he couldn't keep all the terror he felt from bleeding through. Sebastian could see his fear, could practically taste it in the air. He couldn't help but taunt him.  
  
"Hello, Amaimon," Sebastian purred, making the demon king frown, "What are you doing here on Assiah? In the True Cross Academy, no less." He threw a silver knife with lethal accuracy. Amaimon was quick enough to dodge to the side, but he wasn't able to do so without injury. A cut appeared on his cheek, oozing blood too dark to be human.   
  
"Father would be..." Sebastian paused for effect, even as he evaded every single one of Amaimon's attacks with barely any effort, "Displeased, if he hears of your exploits here on Assiah."  
  
"What about you," Amaimon asked flatly, breathing hard as he struggled to stay on top of their battle, "You disappeared for over three hundred years," Three silver daggers went flying over his head as he ducked. Then he threw out his hand, and summoned several hobgoblins to attack Sebastian. "The last time anyone saw you was before the contract with that brat in England."  
  
Sebastian's eyes narrowed, an imperceptible strain affecting his smile. "I have no idea what you're talking about."   
  
The hobgoblins joined together in groups, and tried to overwhelm him by attacking all at once. Like a wave, they crashed over Sebastian and covered him entirely until he could no longer be seen.  
  
"Don't lie, brother." Amaimon jumped to the top of the pile of hobgoblins. He perched on top of the writhing mass and said quietly, "I recognize you." A spear composed of earth and rock appeared in his hands, and he thrust it into the swarm of demons in one swift motion. "Father has been searching for you for centuries."  
  
There was a rumbling coming from underneath Amaimon, and in a rare flash of insight, he leapt off. Just in time. The entire writhing mass went up in a burst of flame, leaving no creature unburnt. Even the ground Sebastian lay on was scorched black. From the ashes he rose, with nothing more than a mildly annoyed expression. His suit however, now bore a large hole through where the spear must have pierced his body. The hole in the fabric opened up to reveal a bloody mess, which was already closing up. Soon enough, only pale white skin could be seen in place of the injury.   
  
"Where I've been is none of your concern, Amaimon." The dying wails of the hobgoblins were excruciating to humans- combined with the extreme heat coming from the flames, it forced Rin and his friends to back up.   
  
Rin held his jaw gently. Bolts of pain were radiating from his chin, and every move he made, hurt. His friends were too stunned to question him; all they could do was witness the fight folding out before them.  
  
"Is that so," Amaimon said, lunging at Sebastian with his claws outstretched, "Iblis?" The name settled into the gloom like a gunshot, stunning everyone present.   
  
The two demons halted their battle momentarily. Sebastian sighed. He didn't look that upset. "How unfortunate. Do you have any idea the lengths I've gone to hide my true identity?" He bent his head from left to right, and closed his eyes. "I suppose there's nothing for it."   
  
When he opened his eyes once more, there were balls of flame dancing in their centers. A faint red glow began to emanate from him. The temperature in the entire area slowly began to rise, a change which did not go unnoticed by the young exwires. Sweat began to drip from their temples, though they were too preoccupied with gaping at Sebastian, now revealed to be Iblis, to notice.  
  
"You should move your friends away, master." Sebastian said, shedding his trench coat. "It might get a bit warm for their taste." With that, he threw his left arm out. Gusts of wind blew past, throwing the exwires back into the forest. Fire then erupted from the ground, erecting a gigantic barrier of flames around him and Amaimon.   
  
Bon was the first to get back on his feet; he immediately ran forwards to return to the clearing they had just been expelled from. He couldn't get close however- the heat from the barrier was too much. Before long, his eyes began to water from the intensity of the heat.   
  
"Stop it, you moron!" Rin suddenly appeared and wrapped his hands around Bon's wrist, trying to pull him back. "You're going to get hurt!"   
  
Furious, Bon shoved him back. To his surprise, Rin actually let go. He even stumbled, and nearly fell to the ground. He only prevented himself from doing so by placing a hand on a nearby tree to steady himself. A pang of guilt streaked through Bon; in the heat of the moment, he had forgotten Rin had been injured.  
  
"Okumura, look, I'm- uh- I didn't" He couldn't apologize. He couldn't find the words.  
  
"Cut it, okay? I don't need an apology," Rin said, leaning heavily on the tree trunk. He glared at Bon through disheveled hair. "Just do me a favor and stay away from Seb and the green bastard."  
  
"Seb?" Bon yelled. The once amusing nickname was now too much for him to handle. He suddenly remembered what he was so upset about. "You're insane! Your 'familiar' is the demon king of fire!" He turned away from the scorching heat and began taking his anger out on Rin. "Do you have any idea how dangerous he is?"  
  
"So?" Rin said, "I don't care. He hasn't done anything bad. He's always helped me!"  
  
Bon resisted the urge to flick Rin's forehead. "That doesn't prove anything. He's still a demon! It's in his nature to be deceptive and evil!"  
  
For a second, Bon thought a flash of hurt appeared in Rin's eyes. It disappeared so quickly he assumed that he imagined it.   
  
"You're wrong." Rin said, facing downwards. He refused to meet Bon's eyes, refused to let Bon see the pain he'd caused with the simple statement. "You're wrong" he repeated.   
  
"We'll see." Bon said, turning away from Rin. He felt like he'd done something wrong- What just happened? Was it something he said? Why was Rin so upset?  
  
Meanwhile, the battle between demons was still going on within the flame barrier.   
  
Amaimon was cornered. Iblis was bearing down on him, a terrifying grin stretching his face as he stared his younger brother down. The earth underneath Iblis's feet melted with each step, leaving distinct bootprints in the ground. Small sparks hovered around him, and whatever foliage that was still miraculously alive would suddenly burn out.  
  
"Please, brother!"Amaimon begged, scrambling backwards. He flinched violently when the flames from the barrier began to snap at his back. He wisely decided to stay still. "Don't kill me!"   
  
Iblis stopped in front of Amaimon. He bent down ever so slowly, his shadow encompassing his brother's entire form. A gloved hand reached out; Amaimon felt his jaw begin to burn with excruciating pain as Iblis held his chin and forced him to look up. "Listen here, little brother, and listen well."   
  
"Stay away from True Cross." His words were slow, and heavy. "My master wishes for you to disappear." Amaimon bit his tongue to stop himself from commenting on Sebastian's casual use of the word master. "Out of the microscopic existing respect I hold for our family bond, I will not disintegrate you on the spot."   
  
"But if I ever see you here again," Iblis trailed off, allowing Amaimon's imagination to do all the threatening for him. He let a few seconds pass, allowing brother to absorb his words. When he was sure that Amaimon had understood, he released his grip on the earth king's jaw and shoved him to the ground.  
  
Like a rodent, Amaimon scrambled to his feet. He was trembling uncontrollably. Iblis snapped his fingers, and a gap appeared in his barrier. "Go," he ordered, and Amaimon wasted no time in escaping.   
  
As soon as the earth king was out of sight, Iblis dispelled his flames barrier. Immediately, the forest returned to its usual gloomy state, albeit with a touch more heat than was normal. It was as if nothing had ever happened. If you could ignore the ashes and the scorched ground, of course.  
  
Sebastian picked up his coat from where he had discarded it. It was perfectly fine, thanks to the protective field he had created in the seconds between removing it and jumping into battle. What? One could rarely get a coat of such high quality in current times. He knew for sure; he had checked nearby clothing establishments, and none of them sold clothing of the same quality as his current outfit.  
  
Once he had finished making himself presentable, he turned to the forest. Time to locate his master and inform him of his success in dispatching Amaimon. He suspected his master and his companions already knew. They would have noticed his barrier disappear.  
  
Sure enough, Rin and the other exwires came stumbling out of the forest. They walked slowly, limbs sore and aching from their impromptu flight. Rin and Izumi took the lead, looking the least affected by the recent events.   
  
Sebastian smiled, and strode forwards. The other exwires flinched back; they stared at him warily as he approached. Almost unconsciously, they moved back with every step he took. His master, however, was a different story.   
  
Rin also walked forwards, moving with a confidence the others had never seen before. There was a glint in his eyes, and Sebastian found himself thinking back to the time Ciel had ordered him to kill those cultists. It was the realization that they had power, real power, in their hands, and all the implications which came along with it. That look in Ciel's eyes- he could see it in Rin now.   
  
How far his new master had come.  
  
The two met at the center, not as allies, as Rin had once called them, but as master and servant. Without hesitation, Sebastian knelt before him. "Your will has been done, master."  
  
"...Look at me." Rin said, and Sebastian slowly raised his head. Their eyes met, blue flames dancing in Rin's eyes, burning brighter than they had ever done before. The fire in Sebastian's eyes dimmed, almost in submission. Something unspoken passed between them. Where before, being Rin's familiar had been nothing more than a facade wherein Sebastian had held all the power; there was now a subtle yet distinct shift in their relationship.  
  
For the first time since he'd summoned Sebastian, Rin had taken the reins. He had given a True Order, pulling on the power of his contract to get Sebastian to do his bidding. He was finally acting as one befitting his station; he was acting like a prince.  
  
"Brother!" A voice called from behind, and the moment was shattered. Rin blinked rapidly, and shook his head. His eyes reverted to their normal state. He was back to being Rin Okumura, the lazy brother and exwire in training. His signature smile slid into place, as if he hadn't just stood before Sebastian as the Prince of Gehenna mere seconds ago.  
  
Sebastian remained kneeling. With significant effort, he reined in his aura and his abilities, until he looked nothing more than an ordinary human once more.   
  
Yukio ran to his brother. Rin turned to him, grinning, but was startled when Yukio grabbed his collar and yanked him backwards. At the same time he pointed his pistol at Sebastian, who did nothing more than smile benignly.  
  
"H-hey! What are you doing?" Rin spluttered. "Sebastian saved all our asses!"  
  
"He's a demon king Rin. He cannot be trusted." Yukio said, keeping his eyes locked on Sebastian. The demon said nothing, electing to keep his gaze trained on Rin.  
  
The other exwires followed Yukio's example, pointing their respective weapons at Sebastian. Izumo and Shiemi's familiars however, completely refused to come near. They eyed him warily, and a single glance from him sent them scurrying away. Only the two tamers' will kept them in Assiah.  
  
"What are you doing here, masquerading as my brother's familiar?" Yukio asked.   
  
Sebastian remained silent. He sent a questioning glance towards Rin, as if to ask the boy if he should answer. He didn't need permission, but the show of submission should be enough to placate the children's fears.  
  
Rin blinked in confusion, unable to understand Sebastian's change in behavior. When the silence continued to drag on, he nodded and said, "Answer my brother."  
  
"Yes, master." Sebastian said, and finally moved his gaze to Yukio. The exorcist resisted the urge to shiver- It was always unsettling to have a demon look you in the eyes. The experience was made all the worse by the sheer power radiating off of Sebastian. There was no hiding it completely anymore; whatever seal had kept his power hidden was now gone. Yukio could sense just how dangerous the demon kneeling before him was. If he wanted, he could destroy everything in this forest with little to no effort. The fact that he wasn't doing so was a mystery, and Yukio hated not knowing.  
  
"My status as your brother's familiar is no masquerade. Nor is it a façade put up to disguise any ulterior motives." He answered clearly. "Your brother and I truly are bound together with a contract," Sebastian lifted his hand to reveal the Faustian mark. "Until said contract is complete, I will remain at his side. Nothing can sever the bond, not without causing significant harm to either of us."  
  
"Being bound to a powerful demon always comes with a price," Izumo muttered from behind the others. There's bitterness in her tone, which she quickly hides by adding, "What price is Rin going to pay for having you as his familiar?"  
  
Sebastian smiled. "The details of our contract are to be kept only between me and my master."  
  
"Why you-!" Bon starts, pushing past Yukio and trying to get to Sebastian. His hands nearly reach the demon's collar, before Rin clutches his arm. The strength of Rin's grip jerks Bon backwards, and he utters a curse.   
  
"Stop!" Rin yells, and pulls at Bon's arm further. He would pull his brother back too, if he wasn't afraid of Yukio actually punching him. There's a tenseness to Yukio's frame that he's seen only a few times before. Once was directly after Father Fujimoto's funeral.  
  
"Show some gratitude! If it wasn't for Seb, we'd all be dead!"  
  
"He's a King of Hell!" Yukio argued.  
  
"He's my familiar!" Rin screamed back, "He hasn't done a single thing to hurt me, or any of you!"   
  
When Rin looked around at his friends, he could see no compassion or acceptance in their faces. His words were of no use. He wouldn't find any help from them. Something cracked deep within him; if this was how they reacted to Sebastian, then he could only dread to think of how they would react if they knew what he was.   
  
"Fine." Rin finally said, choking back tears. "Fine. You don't like Seb? Who fucking cares. Hate him. Hate me." He walked in front of Sebastian and threw his arms out, covering the demon's form with his body. "But I'm not going to let you hurt him. If you want to kill him so badly, then you have to get past me."   
  
Yukio stared at him. What the hell was his brother doing? He was endangering himself, his identity, and everything he had ever worked for. All for the sake of some demon. The others were in equal states of shock, unable to comprehend Rin's choice. How could he choose Sebastian over them?  
  
"That can be arranged." A high, regal, voice says from behind them.   
  
Yukio whirled around, immediately pointing his gun at the newcomer, only to let his hand fall slack once he realizes who was standing in front of him.  
  
Arthur Angel. The current paladin of the True Cross Order.

**Author's Note:**

> Sebastian as Iblis came from me associating Seb with fire after the whole burning-down-the-cultists-incident. Plus, I don't think Iblis's form has been shown in the manga or the anime (from what I remember from a YEAR AND A HALF ago) so it gave me more freedom to take liberties with the character.
> 
> basically i got really lazy about researching details. if ever i continue it, i'll do a better job, i swear
> 
> also! i live off of feedback and constructive criticism, so leave a comment if you have anything (literally anything) to say! i'd love to hear y'alls thoughts and ideas on this


End file.
